Carbon nanotubes (CNTs) and graphene have attracted much attention due to their intrinsic properties as semiconductors as well as their metallic nature. Recently, the use of CNT and graphene as flexible transparent electrodes has been investigated by research and technology groups. Thin films of carbon nanotubes, which are mixtures of semiconducting and metallic nanotubes, are much more resistive than standard indium-doped tin oxide (ITO) at a certain transparency. For this reason, CNT films are doped with many different chemicals, e.g., nitric acid, thionyl chloride, organoantimnoy and the like. Nitric acid and thionyl chloride dopants are effective in reducing the sheet resistivity of CNT thin films but they are not stable and after a few days at room temperature the effect of doping is reduced and sheet resistivity approaches that of un-doped films. Additionally, the use of CNT of graphene as active elements in transistor devices is being investigated. In many cases, the devices will need to be doped to achieve the required transport properties.